Certain communication devices transmit and receive signals over a communication medium, such as a telephone or cable line. These communication devices typically include a transmitter and a receiver (sometimes referred to collectively as a transceiver) coupled to the communication medium through a line interface. Among other things, the line interface generally acts as the electrical interface between the transceiver and the line.
In some communication devices, transmitted signals can be leaked or reflected back to the receiver so as to constitute part of the received signals. For convenience, these reflected signals are often referred to as “echoes.” Similar to a person's voice echoing off of a canyon wall, the echoes in the received signals are generally at a lower power level or intensity than the originally transmitted signals, and there can be multiple echoes corresponding to a single transmitted signal that arrive at the receiver at different times.
The presence of transmit signal echoes in the received signals can limit the dynamic range of the receiver. Therefore, it is preferable to remove, attenuate, or otherwise compensate for the transmit signal echoes in the received signals. Echo cancellation is customarily done by subtracting the transmit amplifier output voltage from the voltage across a matching impedance using complex circuitry such as a hybrid filter network. Among other things, hybrid filter networks are typically expensive in terms of the number of external components, cost, power usage, and printed circuit board (PCB) area.
Some line interface architectures are described in the following references, all of which are hereby incorporated herein by reference in their entireties:
[1] T. M. Rasmus and W. Sylivant, “Balanced Hybrid Circuit,” U.S. Pat. No. 5,822,426, Oct. 13, 1998.
[2] B. Harrington and S. Wurcer, “Broadband Modem Transformer Hybrid,” U.S. Pat. No. 6,163,579, Dec. 19, 2000.
[3] D. V. Gorcea, “Combined Active Impedance and Filter in Line Drivers,” U.S. Patent Application US 2002/0121930 A1, Sep. 5, 2002.
[4] D. M. Joffe, “Method and Apparatus for an Improved Analog Echo Canceller,” U.S. Patent Application US 2002/0126835 A1, Sep. 12, 2002.
[5] F. Sabouri, J. P. Guido and J. G. Kenney Jr., “Line Interface with Gain Feedback Coupled Matching Impedance,” U.S. Patent Application US 2002/0151280 A 1, Oct. 17, 2002.
[6] H. J. Casier et al, “Hybrid Circuit for a Broadband Modem,” U.S. Patent Application US 2002/0176569 A1, Nov. 28, 2002.
[7] F. Sabouri and J. P. Guido, “Line Interface with Second Order High Pass Transfer Function,” U.S. Patent Application US 2003/0109239 A1, Jun. 12, 2003.
[8] T. Blon et al, “Circuit Arrangement for the Analogue Suppression of Echos”, U.S. Patent Application US 2003/0174660 A1, Sep. 18, 2003.